Lightweight silicon-based ceramics are leading candidates to replace heavier nickel-based superalloys for hot section components used in advanced gas turbine engines having increased specific power. Unfortunately, exposures of these materials to the high temperature combustion environments alter the effectiveness of thermally grown silica scales in providing protection from oxidation and component recession during service. Environmental barrier coatings (EBCs) are therefore required that protect the underlying ceramic substrate from environmental attack. Such coatings require good stability in the presence of water vapor, a mechanism for limiting oxygen/water vapor transport and high temperature phase stability. The nature of the silicon-based ceramic recession issue dictates that any EBC system must provide prime reliant performance to ensure full component lifetimes.